Disconnection of a String Carrying Direct Current Power

ABSTRACT

A direct current (DC) power combiner operable to interconnect multiple interconnected photovoltaic strings is disclosed. The DC power combiner may include a device adapted for disconnecting at least one photovoltaic string from the multiple interconnected photovoltaic strings, each photovoltaic string connectible by a first and second DC power line. The device may include a differential current sensor adapted to measure differential current by comparing respective currents in the first and second DC power lines. A first switch is connected in series with the first DC power line. A control module is operatively attached to the differential current sensor and the first switch. The control module may be operable to open the first switch when the differential current sensor measures the differential current to be greater than a maximum allowed current differential, thereby disconnecting the photovoltaic string from the interconnected photovoltaic strings.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to patent application GB1020862.7, filed Dec. 9, 2010, in the United Kingdom Intellectual Property Office. Application GB1020862.7 is herein incorporated by reference.

BACKGROUND

1. Technical Field

This disclosure relates to multiple photovoltaic strings, which have direct current (DC) outputs, which are interconnected at an input of a power combiner junction box and specifically to a system and method to disconnect and or connect a photovoltaic string DC output from the input of the power combiner junction box.

2. Description of Related Art

In a photovoltaic distributed power harvesting system, a photovoltaic string includes a series connection of photovoltaic panels. Photovoltaic strings may be connected in parallel to give a parallel direct current (DC) power output. The parallel DC power output may connect to the input of a direct current (DC) to an alternating current (AC) inverter. The AC power output of the inverter connects across an AC load. The load may be an AC load such as an AC motor or may be an electrical power grid.

A Residual-Current Circuit Breaker (RCCB) is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCCBs are designed to disconnect quickly enough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or short-circuit conditions.

In the United States and Canada, a residual current device is most commonly known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In Australia they are sometimes known as “safety switches” or simply “RCD” and in the United Kingdom, along with circuit breakers, they can be referred to as “trips” or “trip switches.”

Under some circumstances, such as in a photovoltaic distributed power harvesting system, a residual current may also represent a fire hazard.

Thus, there is a need for and it would be advantageous to have a system and method for disconnection of a photovoltaic string carrying direct current power when residual current is detected in the photovoltaic string to prevent fire hazards in photovoltaic arrays.

BRIEF SUMMARY

Various systems and methods are provided for ground fault protection in a photovoltaic power harvesting system. According to some aspects, ground fault protection is provided in a direct current (DC) power combiner, which combines photovoltaic strings to form a photovoltaic array. The detection of a ground fault or residual current in the string causes the string to be disconnected from the photovoltaic array. In some aspects, the possibility for arcing while disconnecting the photovoltaic string from the photovoltaic array is minimized.

According to various aspects, there is provided a direct current (DC) power combiner operable to interconnect multiple interconnected photovoltaic strings. The DC power combiner may include a device adapted for disconnecting at least one photovoltaic string from the multiple interconnected photovoltaic strings, each photovoltaic string connectible by a first and second DC power line. The device may include a differential current sensor adapted to measure differential current by comparing respective currents in the first and second DC power lines. A first switch is connected in series with the first DC power line. A control module operatively is attached to the differential current sensor and the first switch. The control module may be operable to open the first switch when the differential current sensor measures the differential current to be greater than a maximum allowed current differential, thereby isolating the first DC power line from the DC power combiner and disconnecting the photovoltaic string from the interconnected photovoltaic strings.

A second switch may be parallel connected to the first switch to form a first unit. The first unit may be connected in series with the first DC power line. When the first switch is closed, the differential current sensor measures the differential current to be greater than a maximum allowed current differential, then the control module may open the first switch and subsequently may open the second switch. The first switch may allow through substantially more of a current flowing in the first DC power line and the second switch may allow through substantially less of the current flowing in the first DC power line.

A third switch and a fourth switch may be parallel-connected to form a second unit. The second unit may be connected in series with the second DC power line. The third switch may be closed and the fourth switch may be closed, when the differential current sensor measures the differential current to be greater than a maximum allowed current differential, then the control module opens the third switch and subsequently opens the fourth switch. The third switch may allow through substantially more of a current flowing in the second DC power line and the fourth switch may allow through substantially less of the current flowing in the second DC power line.

When the photovoltaic string begins to produce DC power, the first switch may be open and the second switch may be open, the control module closes the second switch and subsequently closes the first switch. The third switch may be open and the fourth switch may be open, the control module closes the fourth switch and subsequently closes the third switch.

The second switch and the fourth switch respectively, may be an insulated gate bipolar transistor (IGBT), an IGBT with integral diode, a solid state switch, metal oxide semiconductor field effect transistor (MOSFET) or a field effect transistor (FET). The first switch and the third switch respectively may be a relay or a circuit breaker.

According to various aspects, there is provided a method for providing ground fault protection. The method performed in a direct current (DC) power combiner operable to interconnect multiple photovoltaic strings, where each photovoltaic string may be connectible by a first and second DC power line. The method measures a differential current by comparing respective currents in the first and second DC power lines. When a control module measures the differential current to be greater than a maximum allowed current differential, a first switch is opened, thereby isolating the first DC power line from the DC power combiner and disconnecting the photovoltaic string from the interconnected photovoltaic strings. The first switch and a second switch may be parallel connected to form a first unit and the first unit may be connected in series with the first DC power line. A third switch and a fourth switch may be connected in parallel to form a second unit and the second unit may be connected in series with the second DC power line. The first unit may be operated simultaneously with the second unit.

When the differential current is measured and found to be greater than a maximum allowed current differential, the third switch may be opened and subsequently the fourth switch opened which isolates the second DC power line from the DC power combiner.

Prior to the comparing of respective currents in the first and second DC power lines, when the photovoltaic string begins to produce DC power with the first switch and the second switch both open. The second switch is closed and subsequently the first switch is closed, thereby connecting the first DC power line to the DC power combiner.

Prior to the comparing of respective currents in the first and second DC power lines, when the photovoltaic string begins to produce DC power with the third switch and the fourth switch both open. The fourth switch is closed and subsequently the third switch is closed, thereby connecting the second DC power line to the DC power combiner. The first switch and the third switch may be operated simultaneously. The second switch and the fourth switch may also be operated simultaneously.

The differential current may be indicative of current between the first DC power line and ground. The differential current may also be indicative of current between the second DC power line and ground. A failure of the string subsequent to the measuring of the differential current may be notified.

A test of the measuring of the differential current may be initiated by injecting a current in the first DC power line prior to and during the measuring. A test of the measuring of the differential current may also be initiated by injecting a current in the second DC power line prior to and during the measuring.

These, additional, and/or other aspects and/or advantages are set forth in the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects are herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 shows a power combiner box according to various aspects.

FIG. 2 shows further details of an isolator and sensing unit according to various aspects.

FIG. 3 shows more details of a digital controller according to various aspects.

FIG. 4 shows a method for disconnecting a string from multiple parallel-connected strings, using an isolation and test unit, according to various aspects.

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying figures.

DETAILED DESCRIPTION

Reference will now be made in detail to various aspects, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout.

Before explaining various aspects in detail, it is to be understood that embodiments are not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

The term “string” or “photovoltaic string” as used herein is a series connection of multiple photovoltaic panels, which may be connected together in parallel to form a “photovoltaic array.”

By way of introduction, various aspects may be directed to a device adapted for a disconnecting of at least one photovoltaic string from multiple interconnected photovoltaic strings. The disconnecting of at least one photovoltaic string may be required because a ground-fault has developed in the at least one photovoltaic string. Minimization of arcing may be performed whilst disconnecting or connecting a direct current (DC) string from multiple interconnected DC strings. Electric arcing can have detrimental effects on electric power distribution systems and electronic equipment. Arcing may occur in switches, circuit breakers, relay contacts, fuses, and poor cable terminations. When a circuit is switched off or a bad connection occurs in a connector, an arc discharge may form across the contacts of relay for example. An arc discharge is an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current flowing through a medium such as air, which may be normally non-conducting. At the beginning of a disconnection, the separation distance between the two contacts may be very small. As a result, the voltage across the air gap between the contacts produces a very large electrical field in terms of volts per millimeter. This large electrical field causes the ignition of an electrical arc between the two sides of the disconnection. If a circuit has enough current and voltage to sustain an arc, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire. The zero crossing of alternating current (AC) power systems may cause an arc not to reignite. However, a direct current system that has DC strings may be more prone to arcing than AC systems because of the absence of zero crossing in DC power systems.

Reference is now made to FIG. 1, which shows a power combiner box 19, according to various aspects. Power combiner box 19 includes multiple combiner circuit boards 12, multiple digital controllers 10, multiple isolation and test units 4, multiple photovoltaic string inputs 8, multiple bus bars 17 and 13, user interface 16, and power supply unit (PSU) 18.

Each combiner circuit board 12 has multiple units 4 a-4 n mechanically mounted to board 12. Unit 4 typically receives a direct current (DC) output from a photovoltaic string 8 or other DC power sources such as a battery, electric fuel cell, or DC generator. Outputs 9 of units 4 a-4 n may be preferably connected in parallel using bus bar 17. Alternatively, outputs 9 of units 4 a-4 n may first be connected to an input of a DC-to-DC converter. An output of the DC-to-DC converter may then be connected to bus bar 17. Multiple bus bars 17 may be further connected in parallel using bus bar 13. Bus bar 13, therefore gives the combined DC power output of power combiner 19. Power combiner 19 may be protected by lightning suppressor 192 (for example a varistor type) and may also be isolated using DC disconnect 194. A digital controller 10 may be operatively attached to units 4 a-4 n via communication and control lines 11. Communication and control lines 11 typically convey control signals to unit 4, to switch on or off unit 4 for example, or to receive signals, which represent currents or voltages measured by sensors located in unit 4 for example.

A user interface 16 may be operatively attached to digital controllers 10 via b-directional communication lines 3. Communication lines 3 may typically be a dual RS-485 bus for example. User interface 16 may be supplied with a DC power from a power supply 18, which converts a mains alternating current (AC) power into the DC power. The DC power may also be used to supply circuit boards 12, controllers 10, and unit circuits 4. Alternatively, circuit boards 12, controllers 10, and unit circuits 4 may be supplied from DC to DC converters, which get an input from strings 8 or the DC from PSU 18.

Reference is now made to FIG. 2, which shows further details of unit 4 according to various aspects. A photovoltaic string 8 has a negative line connected to node X and a positive line connected to one end of a fuse 404. The other end of fuse 404 connects to node A. Across nodes A and X may be a lightening suppressor 402. Suppressor 402 may be additionally connected to electrical earth.

Connected across nodes A and B may be a voltage sensor V1 which provides an output 406. Voltage sensor V1 typically may measure the voltage at node A or node B or the voltage difference between nodes A and B. Output 406 may be operatively attached to controller 10 via control and communication line 11 (not shown). A collector of an insulated gate bipolar transistor (IGBT) Q1 connects to node A. The emitter of Q1 connects to an emitter of an IGBT Q2. The collector of Q2 connects to node B. The emitters of Q1 and Q2 also connect to the anodes of two diodes Dl and D2. The cathode of D1 connects to node A and the cathode of D2 connects to node B. The base of Q1 is connected to the base of Q2. One output of a drive circuit 400 connects to the bases of Q1 and Q2 and another output connects to the anodes of diodes D1 and D2. One side of a relay RR1 contact switch S1 connects to node A. The other side of contact switch S1 connects to one side of a contact switch S2 of relay RR2. The other side of contact switch S2 connects to node B.

Connected across nodes X and Y may be a voltage sensor V2, which provides an output 408. Voltage sensor V2 typically may measure the voltage at node X or node Y or the voltage difference between nodes X and Y. Output 408 may be operatively attached to controller 10 via control and communication line 11. A collector of an insulated gate bipolar transistor (IGBT) Q3 connects to node X. The emitter of Q3 connects to an emitter of an IGBT Q4. The collector of Q4 connects to node Y. The emitters of Q3 and Q4 also connect to the anodes of two diodes D3 and D4. The cathode of D3 connects to node X and the cathode of D4 connects to node Y. The base of Q3 is connected to the base of Q4. One output of a drive circuit 400 connects to the bases of Q3 and Q4 and another output connects to the anodes of diodes D3 and D4. One side of a relay RR3 contact switch S3 connects to node X. The other side of contact switch S3 connects to one side of a contact switch S4 of relay RR4. The other side of contact switch S4 connects to node Y. Relays RR1, RR2, RR3 and RR4 may be typically rated with a breakdown DC voltage of 700 volts for switch contacts S1, S2, S3, and S4. Relays RR1, RR2, RR3, and RR4 may be typically AC relays or DC relays rated at around 1000 volts. During normal operation of strings 8 and combiner box 19, relays RR1, RR2, RR3 and RR4 may be on, i.e. switch contacts S1, S2, S3, S4 may be closed and IGBTs Q1, Q2, Q3, Q4 may be on also. The typical collector emitter voltage (VCE) for an IGBT may be around 2 volts compared to the substantially zero voltage across switch contacts S1, S2, S3, and S4. Therefore, the bulk of the string current (Istring) flows through switch contacts S1 and S2 (in the positive line) and through switch contacts S3 and S4 (in the negative line).

Node B connects to the positive input of residual current device RCD1 and node Y connects to the negative input of residual current device RCD1. Residual current device RCD1 provides a positive line output and a negative line output via a serially connected current sensor R2 in the negative line output. Alternatively residual current device RCD1 may be disposed between the positive and negative outputs of string 8 and nodes A and X. Residual current device RCD1 typically may be Hall Effect residual current device (RCD). Operatively attached to RCD1 may be a test circuit 414 via inductor L2. RCD1 may be operatively attached to test circuit 414 via Hall Effect. Inductor L2 may be connected in series with a battery B1 or a DC power supply from PSU 18, resistor R1 and switch Q5. The gate of switch Q5 may be operatively attached to controller 10 via control and communication line 11. Residual current device RCD1 provides a measure of a differential current between the currents in a positive line of DC output 9 and a negative line of DC output 9. The differential current threshold may be optionally around 20 milliamperes. Additionally residual current device RCD1 provides a measure of a differential voltage between the negative line and positive line of DC output 9. The differential current and the differential voltage may be used to calculate the power of a string 8. The measure of the differential current may be provided to controller 10 via output 410. Output 410 may be operatively attached to controller 10 via control and communication line 11. Output 410 may be provided from the output of an amplifier A1 that has an inductor L1 across an input of amplifier A1. Inductor L1 operatively attaches amplifier A1 to RCD1. A threshold of the differential current to indicate a current leakage may be optionally around a value of 20 milliamperes. The differential current above a value of 20 milliamperes, typically may indicate a current leakage in a photovoltaic string 8. Typically, both a positive and a negative of string 8 may be isolated from electrical earth. The current leakage may be either between the negative and electrical earth or between the positive and electrical earth. The differential current also above 20 milliamperes occurs when for example, an additional current may be imposed onto positive at node B and/or node Y using test circuit 414.

A measure of string 8 current may be also provided from the output 412 of amplifier A2. Output 412 may be operatively attached to controller 10 via control and communication line 11. Output 412 along with voltage sensors 408 and 406 may provide a measure of the power generated by a string 8. Current sensor R2 may be connected to the input of amplifier A2 via series resistors R3 and R4. Current sensor R2 may be located in the positive DC power line or the negative powers line.

Reference is now made to FIG. 3, which shows more details of digital controller 10, according to various aspects. Digital controller 10 includes multiplexors 106 a, 106 b /108 a, 108 b, digital signal processors (DSP) 100 a/100 b, analogue to digital (AD) converters 104 a /104 b, user interface 16, power supply unit 18 and complex programmable logic device (CPLD) 102. User interface 16 may be supplied with direct current (DC) power from PSU 18. User interface 16 may be operatively attached to DSP 100 a and DSP 100 b using bi-directional buses 3, Bus 3 may be typically a dual RS-485 bus. Using 16 photovoltaic strings 8 as an example; preferably DSP 100 a and multiplexors 106 a and 108 a may be responsible for 8 of the photovoltaic strings 8 and DSP 100 b and multiplexors 106 b and 108 b may be responsible for the remaining 8 photovoltaic strings 8. Control line 120 a may be supplied from DSP 100 a to control multiplexor 106 a and control line 120 b may be supplied from DSP 100 b to control multiplexor 106 b. Using the example, multiplexor 106 a receives outputs 406, 408 and 412 for 8 strings 8. Multiplexor 106 a may be controlled by DSP 100 a via control line 120 a to select which string 8 of the 8 strings 8, may be used to provide outputs 406, 408 and 412 to analogue to digital (AD) converter 104 a. Similarly (as multiplexor 106 a) multiplexor 106 b receives outputs 406, 408, and 412 for the other 8 strings 8. Multiplexor 108 a receives output 410 for 8 strings 8. Multiplexor 108 a may be controlled by DSP 100 a via control line 130 a to select which string 8 of the 8 strings 8, may be used to provide output 410 to analogue to digital (AD) converter 104 a. Similarly (as multiplexor 108 a) multiplexor 108 b receives output 410 for the other 8 strings 8. Synchronization between DSP 100 a and DSP 100 b may be by use of bi-directional synchronization control line 132. Complex programmable logic device (CPLD) 102 provides outputs to control test circuit 414, drive circuit 400 and relays RR1-RR4 in each unit 4 used for each string 8. The working operation of DSP 110 b and DSP 100 a may be also verified by CPLD 102 using watchdog bi-directional control lines WDb and WDa respectively. According to another aspect, just one DSP 100 and multiplexors 106 and 108 may be used to implement controller 10 for a number of photovoltaic strings 8.

Reference is now made FIG. 4, which shows a method 401 for disconnecting a string 8 from multiple parallel-connected strings 8 using isolation and test unit 4, according to various aspects. Referring again to FIG. 2, an isolator may be formed (step 403) between nodes A and B, so as to place the isolator in series with the positive power line (step 405) of string 8. Similarly, a second isolator may be formed between nodes X and Y, so as to place the second isolator in series with the negative DC power line of string 8. The isolator and the second isolator may be identical circuits and may be typically activated simultaneously by drive circuits 400. Typically, the negative power line of a string 8 may be not connected to electrical earth. Therefore, operation of both the isolator and the second isolator to disconnect string 8 in the event of a current leakage to earth due to a fault in string 8 prevents the current leakage to electrical earth from other parallel-connected strings 8.

Formation of an isolator in step 403 between nodes A and B includes a first switch and a second switch. The isolator connects and disconnects a string 8 from output 9. The first switch may be connected in parallel with the second switch to form a parallel connection. The parallel connection may be then connected serially between nodes A and B. The first switch may be formed by connecting in series switches S1 and S2 of relays RR1 and RR2 respectively. The second switch includes a collector of Q1 connected to node A and a collector of Q2 connected to node B. Emitters of Q1 and Q2 may be connected together. Where the emitters of Q1 and Q2 may be connected together may be also connected the anodes of diodes D1 and D2. The cathode of D1 connects to node A and the cathode of D2 connects to node B. The bases of Q1 and Q2 may be connected together and where the bases of Q1 and Q2 may be connected together, a connection to drive circuit 400 may be made. A second connection to drive circuit 400 may be made where the emitters of Q1 and Q2 may be connected together.

An input of residual current device RCD1 may be connected across nodes B and Y. RCD1 provides a measure (step 407) of a level of differential current between current flowing in the positive line of string 8 and the current flowing in the negative line of string 8. A differential current, which may be greater than a predetermined value, may be typically indicative of leakage current to electrical earth owing to fault in a string 8 or power lines connected to string 8. Also, a differential current, which may be greater than a predetermined value, may be provided by test circuit 414 so as to ensure that measurement step 407 may be functioning correctly as part of periodic test function. The level of differential current may be measured when the isolation between nodes A and B/X and Y may be ON, in a normal mode of operation. During the normal mode of operation, current in the positive line may be the sum of current flowing through switch contacts S1, S2 and current flowing through Q1 and D1 and Q2 and D2. During the normal mode of operation, current in the negative line may be the sum of current flowing through switch contacts S3, S4 and current flowing through Q3 and D3 and Q4 and D4. The lower ON resistance of switches S1-S4 means that the current going through switches S1-S4 may be much greater than the current going through IGBTs Q1-Q2 and diodes D1-D2. During the normal mode of operation, the level of differential current between current flowing in the positive line of string 8 and the current flowing in the negative line of string 8 may be substantially zero and/or less than 20 milliamperes. In decision box 409 if the modulus of the level of differential current may be substantially zero and/or less than 20 milliamperes, monitoring of the differential current continues with step 407. In decision box 409 if the modulus of the level of differential current may be greater than a predetermined value (typically greater than 20 milliamperes.), relays RR1-RR4 may be switched OFF (step 411) thereby opening switches S1-S4. The opening of switches S1-S4 substantially increases the current IGBTs Q1-Q4 and diodes D1-D4, which may be still ON. Substantial increase in the current through IGBTs Q1-Q4 and diodes D1-D4 means that the opening of switches S1-S4 in step 411 allows for minimized arcing of switches S1-S4. After switches S1-S4 may be opened, IGBTs Q1-Q4 may be turned OFF (step 413).

In decision 415, a check may be made to see if IGBTs Q1-Q4 and switches S1-S4 may be indeed turned OFF by measuring the voltages across nodes A, B and nodes X, Y. The voltages across nodes A, B and nodes X, Y may be measured by voltage sensor V1 and voltage sensor V2 respectively. Voltage sensor V1 provides output 406 and voltage sensor V2 provides output 408. Additionally current and voltage sensing from outputs 410, 412 and RCD1 may be used to see if IGBTs Q1-Q4 and switches S1-S4 may be indeed turned OFF. If IGBTs Q1-Q4 and switches S1-S4 may be indeed turned OFF a disconnected status for string may be initiated (step 419), otherwise an alarm or indication of a fault may be made with step 417.

Connection of a string 8, for example when multiple strings 8 begin to generate DC power, has IGBTs Q1-Q4 and switches S1-S4 initially turned OFF. First, IGBTs Q1-Q4 may be turned on, followed by switches S1-S4 being closed. Turning on IGBTs Q1-Q4 first before switches S1-S4 being closed prevents arcing of switches S1-S4. During a normal operation of a string 8, the lower ON resistance of switches S1-S4 means that the current going through switches S1-S4 may be much greater than the current going through IGBTs Q1-Q2 and diodes D1-D2.

The term “comprising” as used herein, refers to an open group of elements for example, comprising an element A and an element B means including one or more of element A and one or more of element B and other elements other than element A and element B may be included.

The terms “sensing” and “measuring” are used herein interchangeably.

The indefinite articles “a”, “an” is used herein, such as “a string”, “a switch” have the meaning of “one or more” that is “one or more strings or “one or more switches.”

Examples of various features/components/operations have been provided to facilitate understanding of various embodiments. In addition, various preferences have been discussed to facilitate understanding of the disclosed aspects. It is to be understood that all examples and preferences disclosed herein are intended to be non-limiting.

Although selected aspects have been shown and described individually, it is to be understood that at least aspects of the described aspects may be combined. Also, although selected aspects have been shown and described, it is to be understood that other embodiments are not so limited. Instead, it is to be appreciated that changes may be made to these aspects without departing from the principles and spirit of the disclosure. 

1. A direct current (DC) power combiner including: a differential current sensor adapted to measure differential current of at least one photovoltaic string of a plurality of photovoltaic strings, the differential current measured by comparing respective currents in first and second DC power lines that interconnect the at least one photovoltaic string to the plurality of photovoltaic strings; a first switch connected in series with the first DC power line; and a control module operatively attached to the differential current sensor and the first switch, wherein said control module is operable to open the first switch when the differential current sensor measures the differential current to be greater than a predetermined current differential, thereby preventing current flow between at the least one photovoltaic string to the first DC power line.
 2. The DC power combiner of claim 1, further comprising a second switch parallel connected to said first switch to form a first unit, wherein the first unit is connected in series with the first DC power line.
 3. The DC power combiner of claim 2 wherein, when the first switch is closed and the differential current sensor measures the differential current to be greater than the predetermined current differential, the control module opens the first switch and subsequently opens the second switch.
 4. The DC power combiner of claim 2 wherein the first switch allows through substantially more of a current flowing in the first DC power line and the second switch allows through substantially less of the current flowing in the first DC power line.
 5. The DC power combiner of claim 1, further comprising: a third switch and a fourth switch parallel-connected to form a second unit; wherein the second unit is connected in series between the at least one photovoltaic string and the second DC power line; and wherein, when the third switch is closed and the fourth switch is closed and the differential current sensor measures the differential current to be greater than the predetermined current differential, the control module opens the third switch and subsequently opens the fourth switch.
 6. The DC power combiner of claim 5 wherein the third switch allows through substantially more of a current flowing in the second DC power line and the fourth switch allows through substantially less of the current flowing in the second DC power line.
 7. The DC power combiner of claim 2 wherein, when the photovoltaic string begins to produce DC power and the first switch is open and the second switch is open, the control module closes the second switch and subsequently closes the first switch.
 8. The DC power combiner of claim 5 wherein, when the third switch is open and the fourth switch is open, the control module closes the fourth switch and subsequently closes the third switch.
 9. The DC power combiner of claim 2 wherein said second switch is selected from a group consisting of an insulated gate bipolar transistor (IGBT), an IGBT with integral diode, a solid state switch, metal oxide semiconductor field effect transistor (MOSFET), and a field effect transistor (FET).
 10. The DC power combiner of claim 5, wherein said fourth switch is selected from a group consisting of an insulated gate bipolar transistor (IGBT), an IGBT with integral diode, a solid state switch, metal oxide semiconductor field effect transistor (MOSFET), and a field effect transistor (FET).
 11. The DC power combiner of claim 1, wherein said first switch is selected from a group consisting of a relay and a circuit breaker.
 12. The DC power combiner of claim 5, wherein said third switch is selected from a group consisting of a relay and a circuit breaker.
 13. A method including the steps of: measuring a differential current of at least one photovoltaic string of a plurality of photovoltaic strings, the differential current measured by comparing respective currents in first and second DC power lines that interconnect the at least one photovoltaic string to the plurality of photovoltaic strings; and upon a control module measuring the differential current to be greater than a predetermined current differential, opening a first switch connected in series between the at least one photovoltaic string and the first DC power line, thereby preventing current flow between the at least one photovoltaic string to the first DC power line.
 14. The method of claim 13 wherein a third switch and a fourth switch are parallel-connected to form a second unit series connected between the at least one photovoltaic string and the second DC power line, the method further comprising: upon measuring the differential current to be greater than the predetermined differential, opening the third switch and subsequently opening the fourth switch.
 15. The method of claim 13 wherein the first switch and a second switch are parallel connected to form a first unit series-connected between the at least one photovoltaic string and the first DC power line, the method further comprising: when the at least one photovoltaic string begins to produce DC power and when the first switch and the second switch are both open, closing the second switch and subsequently closing the first switch.
 16. The method of claim 14, further comprising: when the photovoltaic string begins to produce DC power and the third switch and the fourth switch are both open, closing the fourth switch and subsequently closing the third switch.
 17. The method of claim 13, wherein the differential current is indicative of current between the first DC power line and ground.
 18. The method of claim 13 wherein the differential current is indicative of current between the second DC power line and ground.
 19. The method of claim 13, further comprising: initiating a test of said measuring by injecting a current in at least one of said first and said second DC power lines prior to and during said measuring.
 20. The method of claim 14 wherein said first switch and said third switch are operated simultaneously and said second switch and said fourth switch are operated simultaneously. 